In some military vehicles the main gun is mounted in a turret which rotates around a vertical axis in the azimuth plane; the gun is mounted on the turret for swinging movement in a vertical elevational plane. The gun is trained on the enemy target (airborn or land) by combinational movements of the turret and gun in the azimuth and elevation planes.
In one particular vehicle the minimum gun depression angle is -20.degree. except over the rear end of the vehicle where the elevation angle is restricted to -7.degree.. An aim of this invention is to provide circuitry that enables the gunner to have tracking capability in the low level zones between -7.degree. and -20.degree. except in the so-called "avoidance zone" defined by the vehicle rear end. As the gun approaches the avoidance zone a "pop-up" command is generated by circuit components that respond to azimuth train velocity and gun position. The pop-up command is applied to relay contacts shunted around resistances in a potentiometer that is responsive to gun motion in the elevational plane.
Potentiometer voltage signal is summed with an elevation tachometer-derived voltage to limit the normal maximum elevation rate from 90.degree./sec at -5.degree. to 0.degree./sec at -20 .degree. elevation. In the avoidance zone this limit is modified to 90.degree./sec allowable at +18.degree. and 0.degree./sec at -7.degree.. This function is accomplished by switching the voltage levels on the elevation position limit potentiometer as a function of azimuth position and velocity.
As the gun approaches avoidance zone 14 below the -7.degree. elevation it is necessary for safety reasons to limit possible azimuthal acceleration inputs by the gunner. The circuitry senses and stores the difference between the so-called "safe" azimuthal rate and the actual azimuth rate. When a pop-up command is issued the stored difference provides a signal that limits the azimuthal velocity to a safe value until the gun is elevated to the -7.degree. elevation.